1. Field of the Invention
This invention relates to a safety device for an electric shutter in a camera having a focal plane shutter which renders shutter release impossible when the power supply battery in the camera has been consumed.
2. Description of the Prior Art
In the electric shutter, when the battery has been consumed, the rearward curtain of the focal plane shutter is no longer subjected to restraint and the shutter moves without forming the slit, thus failing to provide a desired shutter speed.
It is therefore required that when the power supply battery has been consumed, the shutter be prevented from being actuated even if one tries to effect shutter release.
For this purpose, it has been known to utilize a forward curtain restraining member for restraining the forward shutter curtain in charged condition or an anticipatory actuation member starting to move prior to movement of the forward shutter curtain and to effect engagement of such member only when the battery has been consumed, thereby inhibiting such restraint from being released. However, the technique utilizing such a relation of engagement has suffered from shocks of engagement resulting from a rapidly moving body being abruptly restrained, and has been disadvantageous in respect of durability of various parts as a result of such shocks.
Further, once the anticipatory actuation member is restrained in the course of its movement, it can no longer expect much of an inertia force when its movement is again started to release the forward curtain restraining member and thus, it is necessary, in advance, to increase the releasing force of the anticipatory actuation member, which in turn requires an increased winding force and may result in reduced durability of such member. Moreover, in the electric shutter, it takes some time for the magnet to be energized and this leads to the necessity of compensating for the time delay, which also makes it necessary to adopt a signal from a switch operatively associated with the movement of the anticipatory actuation member. This also requires the releasing force of the anticipatory actuation member to be increased and thus, this member must be heavier in weight in order that it may remain operative for a predetermined time, which would in turn lead to larger sizes of the parts.
It is also known to use a further magnet, in addition to the rearward curtain restraining magnet, to inhibit shutter release, but this has been disadvantageous in that the service life of the battery is shortened.